This application is based on Patent Application No. 11-147170 (1999) filed May 26, 1999 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a semiconductor optical pulse compression waveguide for compressing optical pulses to generate narrower width optical pulses applicable for optical communication, optical memory, optical measurement and the like, and more particularly to a technique for transforming originally narrow width optical pulses, which are output from a Q-switched semiconductor laser, gain-switched semiconductor laser or mode-locked semiconductor laser, to ultrashort optical pulses.
2. Description of the Prior Art
Conventionally, several methods have been proposed and available as optical pulse compression technique. A first one of them utilizes nonlinearity of an optical fiber (see, Reference 1; J. T. Ong, R. Takahashi, M. Tsuchiya, S. H. Wong, R. T. Sahara, Y. Ogawa and T. Kamiya, xe2x80x9cSubpicosecond Soliton Compression of Gain Switched Diode Laser Pulses Using an Erbium-Doped Fiber Amplifierxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 29, No. 6, June 1993, and the like). The nonlinearity of the optical fiber utilized in this method refers to such features as optical group velocity dispersion in the optical fiber and optical intensity dependence of the refractive index of a material constituting the optical fiber.
The optical group velocity dispersion refers to a property of varying optical group velocity in a material. Utilizing this property can shorten the width of the optical pulses in chirping, for example, where chirping refers to a phenomenon in which the wavelength of an optical pulse varies from its front to rear portion.
More specifically, consider an optical fiber with smaller group velocity for the front wavelength and greater group velocity for the rear wavelength, in which case optical pulses can have a shorter front wavelength and a longer rear wavelength. During the propagation of these optical pulses through the optical fiber, the rear portions of the optical pulses gradually catch up with the front portions because of the greater rear group velocity, resulting in gradual reduction in the width of the optical pulses. Thus, the optical pulses with such a property, chirping, can be compressed during the propagation through the optical fiber.
Alternatively, it is also possible to utilize the optical intensity dependence of refractive index of the material constituting the optical fiber. This method utilizes the difference in the refractive index for the intensity of light. Using this effect (optical intensity dependence of refractive index) can narrow or widen, or change the pulse width by selecting conditions during the propagation of the optical pulses through a fiber. The effect generally grow stronger with an increase in the energy of the optical pulses. Taking account of this, an actual system amplifies the optical pulse energy by passing through an erbium-doped fiber amplifier or the like, followed by the compression of the optical pulses through an optical fiber.
A second method relates to an optical waveguide pulse compression device utilizing the group velocity dispersion in a waveguide. It also uses the wavelength dependence of the optical group velocity as the foregoing method utilizing the optical fiber.
The waveguide is fabricated using a dielectric or a semiconductor. The mechanism for compressing optical pulses utilizing the group velocity dispersion of a waveguide is principally -the same as the method utilizing the group velocity dispersion in the optical fiber (see, Reference 2; Y. Lee, xe2x80x9cPulse compression using coupled-waveguide structures as highly dispersive elementsxe2x80x9d, Applied Physics Letters, Vol. 73, No. 19, November 1998 and the like). However, the waveguide using the semiconductor or dielectric has a higher degree of flexibility in the designing for its fabrication. Thus, taking advantage of this feature with devising the structure of the waveguide can increase the variations in the group velocity associated with the changes in the wavelength. The length of the pulse compression device can be reduced, as well. For example, the length from several tens of meters to hundreds of meters required for the optical fiber can be reduced to a few millimeters to centimeters by using the waveguide.
All the foregoing conventional methods mainly utilize the property that the wavelength of the optical pulses varies from the front portion to the rear portion, that is, chirping. A more important thing is that the manner of the chirping must well match the dispersion characteristic of the optical fiber or of the optical waveguide. Although the wavelength varies in the optical pulses, sufficient effect cannot be achieved if the manner of the variations in the wavelength does not match the characteristic of the group velocity dispersion of the optical fiber. For example, the optical pulses having a short wavelength at their front portion, long wavelength at their central regions and again short wavelength at their final regions cannot cause the pulse compression substantially, and hence cannot be utilized. Furthermore, these conventional methods are inapplicable to the compression of wide optical pulses without chirping.
From a practical point of view, it is strongly desired that the components used for the pulse compression are small in size. However, the optical fiber used for the pulse compression is hundreds of meters to several kilometers long, occupying a large space. In addition, the optical amplification carried out by the erbium-doped fiber in advance requires an erbium-doped fiber, a light source for exciting the erbium-doped fiber, a coupler for interconnecting the fibers and the like, which inevitably increases the size of the entire system, and presents a great drawback to actual use. In addition, it is necessary for achieving the desired pulse compression to adjust the length of the fiber in accordance with the chirping, which limits a degree of flexibility in designing, fettering the application to commercial systems.
Although the conventional optical waveguide pulse compression device can be made much smaller than the compression system using the optical fiber, it is not small enough to meet the requirement of practical systems. Practical systems usually employ as their optical pulse source a gain-switched semiconductor laser or Q-switched semiconductor laser light source. The width of the optical pulses generated by these semiconductor lasers are about 10 ps wide, and the conventional optical waveguide pulse compression device requires a waveguide of a few centimeter long to compress the optical pulses on the order of 10 ps wide. In addition, the structure of the conventional optical waveguide pulse compression device greatly differs from that of the conventional semiconductor laser (see, Reference 2; Y. Lee, xe2x80x9cPulse compression using coupled-waveguide structures as highly dispersive elementsxe2x80x9d, Applied Physics Letters, Vol. 73, No. 19, November 1998 and the like). Thus, to integrate the conventional optical waveguide pulse compression device into the same semiconductor substrate as the Q-switched semiconductor laser or gain-switched semiconductor laser, a complicated process is required, presenting a problem to implement practical systems.
Therefore, an object of the present invention is to provide a semiconductor optical pulse compression waveguide with a novel structure that can greatly reduce its size as compared with the conventional optical pulse compression system, and that can simplify its fabrication, and particularly, to provide a semiconductor optical pulse compression waveguide with a novel structure that can be easily integrated into the same semiconductor substrate as a Q-switched semiconductor laser or gain-switched semiconductor laser used as an optical pulse light source, and that can be manufactured without using a complex fabrication process.
Another object of the present invention is to provide a novel structure semiconductor optical pulse generating laser with an optical pulse compression waveguide that can integrate into the same substrate the semiconductor optical pulse compression waveguide and one of various types of semiconductor lasers serving an optical pulse light source, thereby obtaining ultrashort optical pulses.
The inventors of the present invention accomplished the present invention as a result of intensive research into an optical pulse compression method based on a new operation principle, and into a component structure for realizing the operation principle, in which they found that the pulse width can be reduced step by step by passing optical pulses alternately through a saturable absorber and a gain region.
According to the present invention, there is provided a semiconductor optical pulse compression waveguide composed of a semiconductor optical waveguide that guides an optical signal and includes an optical pulse compression region, the optical pulse compression region comprising: at least one saturable amplifier region that loses its amplification characteristic after amplifying a fixed amount of optical energy; and at least one saturable absorber region that loses its absorption characteristic after absorbing a fixed amount of optical energy, wherein the at least one saturable amplifier region and the at least one saturable absorber region are disposed alternately in series in the optical compression region.
The semiconductor optical pulse compression waveguide can compress the width of the optical pulse injected from the outside while the optical pulse passes through the optical pulse compression region in the course of traveling through the optical waveguide.
The semiconductor optical pulse compression waveguide may further comprise a first light modulation region disposed between an optical pulse inlet, through which the optical signal is injected into the optical waveguide, and the optical pulse compression region, for regulating energy of an optical pulse. Alternatively, it may further comprise a second light modulation region disposed between the optical pulse compression region and an optical pulse outlet, through which the optical signal is extracted from the optical waveguide, for regulating energy of an optical pulse. Furthermore, it may comprise both the optical pulse outlet side light modulation region and the optical pulse inlet side light modulation region.
According to the present invention, the semiconductor optical pulse compression waveguide can be combined with a semiconductor laser serving as a pulse light source to implement a device for generating ultrashort pulses. In particular, it is preferable that the semiconductor laser be integrated with the semiconductor optical pulse compression waveguide on the same semiconductor substrate.
According to the present invention, a semiconductor optical pulse generating laser with the optical pulse compression waveguide can be implemented by combining the semiconductor optical pulse compression waveguide with a semiconductor laser capable of generating optical pulses using such a technique as Q-switching, gain switching and mode locking. In other words, the semiconductor laser can comprise in its resonator the foregoing semiconductor optical pulse compression waveguide structure, and generate optical pulses using the technique such as Q-switching, gain switching and mode locking.
More specifically, the semiconductor laser can comprise to generate optical pulses a semiconductor laser region for generating optical pulses using such a technique as Q-switching, gain switching and mode locking, and a semiconductor optical pulse compression waveguide region in accordance with the present invention with the foregoing structure, which is disposed in series on an extension of the resonator of the laser, wherein the optical pulses generated by the semiconductor laser region is output after passing through the semiconductor optical pulse compression waveguide region. It is preferable that the semiconductor laser region and the semiconductor optical pulse compression waveguide region share the waveguide structure with each other, in which they are integrated in the same semiconductor substrate.
According to the present invention, the semiconductor optical pulse compression waveguide has an advantage of being able to compress the optical pulses without increasing the device length so much because it compresses the optical pulses step by step by iterating partial optical absorption and re-amplification using the alternately disposed saturable absorption regions and gain regions. In addition, it offers a practical advantage of being able to compress the optical pulses independently of wavelength distribution or fluctuations in the optical pulses, or of the presence or absence of the chirping. Furthermore, because its structure such as the layer structure of the optical pulse compression waveguide can be substantially the same as that of the semiconductor laser, the optical pulse compression waveguide and the semiconductor laser used as the light source can be integrated into the same substrate, simplifying the fabrication process. Moreover, taking advantage of being able to integrate into the same substrate, it is not difficult to implement a semiconductor optical pulse generating laser with the optical pulse compression waveguide, in which the optical pulse compression waveguide structure is disposed in the resonator of the semiconductor laser acting as the light source. The semiconductor optical pulse generating laser with such a structure is suitable for generating ultrashort optical pulses.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.